This invention relates to liquid crystalline compositions comprised of phyllosilicates and polymers. More particularly, this invention relates to liquid crystalline compositions for use in barrier applications.
It is common practice to blend materials such as mica, talc, kaolin, precipitated calcium carbonate, precipitated silica, fumed silica, barite, zinc oxide, carbon black, etc. into elastomeric, thermoset, and thermoplastic polymers. Inorganic fillers are added as high as 40 to 50 weight percent. The addition of minerals to polymers can improve properties such as strength, stiffness, temperature and impact resistance, dimensional stability, and scratch resistance. In conventional mineral/polymer composite materials, the mineral phases are dispersed within the polymer matrix at the micrometer scale.
Much interest has been created by the more recent advance of producing nanocomposites. Nanocompositesxe2x80x94nanometer sized dispersions of organophilic clays in polymers to form polymeric hybridsxe2x80x94have been demonstrated to produce dramatic improvements in mechanical properties, heat resistance, thermal stability, and reduced gas permeability of the base polymer without loss of impact strength. Due to their enhanced barrier properties and clarity, nanocomposites are well suited for use as gas transport barriers in packaging applications. Examples include nylon-based nanocomposites for food and beverage packaging which incorporate the nanocomposite layer within single or multi-layer films. Reduction in gas diffusion is attributed to the presence of the clay particles which act to increase diffusion path length. Current nanocomposites characteristically contain small amounts of phyllosilicates dispersed in the base polymer, typically six percent or less, producing overall improvements in reduction of gas transfer that can be calculated from simple diffusion theory and which depend on the generation of a tortuous diffusion path originating from the presence of the dispersed organoclay. A major impediment to the commercial development of nanocomposites has been the difficulty of producing homogenous dispersions of organoclays within the polymer matrix. To improve the affinity between the hydrophilic clay surface and organic polymers, clays are treated by cation exchange with high-molecular-weight onium salts (e.g., ammonium, phosphonium, and sulfonium). However, even with surface treatment, phyllosilicates can still only be dispersed at the nanoscale into polymers that contain polar functional groups. The presence of these polar functional groups makes high barrier polymers, such as PET, EVOH, and Nylon, sensitive to water, thus requiring their use as multiplayer laminates which contain an external, water-barrier layer. The requirement of multiplayer laminates thus increases manufacturing costs of flexible packaging films.
Accordingly, there is a continuing need to provide low cost materials which provide superior barriers against gas transport and diffusion.
One embodiment of the present invention provides a phyllosilicate-polymer composition comprised of a phyllosilicate and a polymer layer adsorbed onto the basal surface of the phyllosilicate providing a phyllosilicate-polymer composition. In the phyllosilicate-polymer composition the phyllosilicate-polymer composition is present as a single phyllosilicate-polymer phase and exhibits an anomalous basal spacing. Preferably a sufficient proportion of exchange sites on the basal surface of the phyllosilicate are substantially occupied by protons. In other embodiments of the invention the polymer has at least one hydroxyl group and can be selected from the group consisting of polyethylene glycol, polypropylene glycol and their monoalkyl ether derivatives. Still other embodiments of the invention provide phyllosilicate-polymer compositions wherein the polymer comprises greater than 27 weight percent of the phyllosilicate-polymer composition, the basal surface of the phyllosilicate is bound substantially with hydrogen ions or the basal spacing of the phyllosilicate-polymer composition increases as the molecular weight of the polymer increases. In yet another embodiment, the basal spacing of the phyllosilicate-polymer composition is equal to or greater than 17.8 xc3x85.
Still another embodiment of the present invention provides an anisotropic liquid crystalline composite comprising a phyllosilicate-polymer composite made of at least a phyllosilicate, and a polymer adsorbed onto the basal surface of the phyllosilicate. In this embodiment the phyllosilicate-polymer composite has a highly ordered, well-defined basal spacing and the phyllosilicate-polymer composition is birefringent. Other aspects of this embodiment of the invention include a nematically oriented phyllosilicate in the phyllosilicate-polymer composition. In this embodiment, the phyllosilicate can make up greater than 10 percent of the phyllosilicate-polymer composite. Typically, the phyllosilicate is selected from the group consisting of kaolins, talcs and montmorillonites and the polymer is water soluble. In another embodiment the polymer can be hydrophobic, such as polyethylene. The anisotropic liquid crystalline composite of this embodiment can further comprise an antioxidant. In still another embodiment of the invention the anisotropic liquid crystalline comprises a barrier layer such that the barrier layer provides a gas permeability below the gas permeability of the polymer alone.
The present invention also provides methods for producing an anisotropic liquid crystalline composite from a phyllosilicate and a polymer. The method can include the steps of suspending a phyllosilicate in a compatible solvent, dissolving a polymer that is soluble in the compatible solvent, and removing a sufficient amount of the compatible solvent to produce an anisotropic liquid crystalline composite. In the method the solvent can be water and the polymer polyethylene glycol. The method can further include the step of purifying the phyllosilicate prior to suspending the phyllosilicate in the compatible solvent. The method can provide an anisotropic liquid crystalline composition comprising between about 10 and 70 percent phyllosilicate. The method also provides composites which are extrudable and useful as gas barrier layers.
In yet another embodiment of the present invention, a barrier film for use in packaging and coating applications is provided having reduced gas permeability. The barrier film comprises an anisotropic liquid crystalline composite layer having a gas permeability below the permeability of the polymer alone. Typically the film is transparent and is comprised of a phyllosilicate and a polymer or a combination of polymers. The phyllosilicate can make up greater than ten percent by weight of the liquid crystalline composite layer. The barrier film of the present invention can also be incorporated into other films as a barrier layer to form a multilayer film. As a non-limiting example, the liquid crystal composite can be blended with polyethylene to impart water barrier properties and improve extrusion properties.
The above described embodiments are set forth in more detail in the following description and illustrated in the drawings described hereinbelow.